Link State Routing Protocol

Link State Routing Protocol:
Link mean an interface on a router and state is the information and state of the links. Information and state of an interface include
ü  IP address
ü  Subnet mask
ü  Type of network
ü  Cost associated with link
ü  Neighboring routers on the link

In Link state routing protocol state of any link is forward to neighbor, through which every router create a full map of network. Link state use the multicast instead of broadcast. In packet switching network there are two link state routing protocol i.e. Open Shortest Path First (OSPF), Intermediate System-Intermediate System (IS-IS)

How Link state routing protocol works:
1.       Link state process starts with learning it’s directly connect router.
2.       Link state routers exchange hello packet and Connected link state routers
3.       Each router builds its own Link State Packet (LSP) which includes information about neighbors such as neighbor ID, link type, & bandwidth and floods it to all neighbors who then store the information and then forward it until all routers have the same information.
4.       Routers use LSP database to build a network topology map & calculate the best path to each destination.
LSPs are sent out under the following conditions:
        i.            When routing process starts
      ii.            When there is a change in topology
Every router that connects to a given OSPF area should learn the exact same topology data. Each router creates a link state database (LSDB) using LSAs. Then, the router uses the Shortest Path First (SPF) algorithm to find the best path.

Determining the shortest path
The shortest path to a destination determined by adding the costs & finding the lowest cost

link state routing protocol Requirements:

Typically link state routing protocols use more memory, CPU processing and need lots of bandwidth for Initial startup of link state routing protocols.

Distance Vector Routing Protocols definition

Distance Vector means ‘distance to final destination’ and ‘Vector, or direction, traffic should be directed’. 
In Distance vector protocol router only knows its neighbour, but it does not know where that neighbor learned the route. Routing Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP) are famous distance vector protocols.
The main Characteristics of Distance Vector routing protocols are
        i.            Periodic updates
      ii.            Neighbors
    iii.            First time use Broadcast updates, send updates on (Entire routing table is included with routing update)
In distance vector routing routers do not have knowledge of the entire path to a destination, just knows it’s connect neighbor. 

Advantages of distance vector protocol:
        i.            Distance vector protocol are simple to implement normally you don’t need much expertise to implement this.
      ii.            Usually Distance vector routing protocols don’t required costly and hardware with high performance.    
Disadvantages of distance vector protocol:
        i.            Distance vector protocols have slow convergence as compare to link state routing protocols.
      ii.            Normally distance vector protocols are suitable for small networks.
    iii.            Most distance vector routing protocols take the very simple approach of telling their neighbors everything they know by broadcasting their entire route table.
     iv.            Routing loops can occur in distance vector protocols because of slow convergence, although there are some methods that are used for avoiding loop in these protocols.

How distance vector routing protocols works:

        i.            In network discovery process every router adds its connected network to its routing tables. And send the information about the connected networks to its neighbours. For example in below figure R0 send the info about its connect networks to,, and to R1 and vice versa in case of R1.

      ii.            Routers will exchange routing information and routing updates received from other routers
    iii.            If there is new information then Metric is updated new information is stored in routing table
     iv.            Once the process is completed all routing tables in the network contain the same network information. You can see in figure that each router have all networks in its routing table.

Routing loops can be occur in distance vector (need modifications)
Routing loops are a condition in which packets continuously traverse a network. Routing loop effects networks performance by use of bandwidth and extra CPU usage. You can minimize routing loops by defining maximum hop count, hold-down timers, split horizon, route poisoning and triggered updates.

  • Causes of routing loops
        Incorrectly configured static routes
        Incorrectly configured route redistribution
        Slow convergence
        Incorrectly configured discard routes

Examples of Distance Vector Routing Protocols
      RIP (Routing Information Protocol)
RIP is one of famous DV routing protocol. RIP sending out periodic updates. Using 4 different timers to RIP ensure information is accurate and convergence is achieved in a timely manner

·        EIGRP (Enhanced Interior Gateway Routing Protocol)
A distance vector routing protocols that has some features of link state routing protocols
EIGRP is a Cisco proprietary routing protocol. EIGRP sending out triggered updates